The invention relates to a vent-type plastication molding machine such as a vent-type injection molding machine and a vent-type extrusion molding machine and in particular to the feeding of material to such machine.
According to the prior art, a vent-type injection molding machine has a heating cylinder provided with a vent for degassing. The heating cylinder houses a screw having a first stage of a feed zone, a first compression zone, and a first metering zone, and a second stage of a deep grooved portion following the first stage, a second compression zone, and a second metering zone. Such a two-stage screw is rotatably provided in the heating cylinder so as to reciprocally slide in an axial direction of the cylinder. The material is melted under pressure with volatile gas confined therein at the first stage of the screw. The material then enters the deep grooved portion of the second stage which is either open to the atmosphere or under pressure lower than that at which the first stage is kept, resulting in inflation and foaming of the material. The molten material or resin is conveyed along the deep grooved portion of the screw, releasing gas which is discharged through the vent. In a vent-type plastication molding machine, the screw makes a reciprocal sliding motion in an axial direction with regard to the heating cylinder in addition to the intermittent motion of rotation interrupted by stillness at intervals. It is known that a vent-type extrusion molding machine suffers occasional clogging of its vent with the result that gas exhaustion is made impossible. The same is true of a vent-type injection molding machine. Clogging of the vent occasionally takes place at a standstill of the screw when the molten material inflates due to the gas therein or when the molten material located at the first stage of the screw overflows, causing gas to be generated within the material so late that the material flows into the vent before the gas breaks the resin film to escape from it. In the injection stroke, the molten material located by the vent is pushed up above the screw flight at the nozzle side of the lower end of the vent due to the relative motion of the screw with regard to the inside wall of the cylinder as the screw moves toward the nozzle. The molten material is further raised from the lower end to the upper end of the vent, sometimes flowing over the upper edge of the vent, even more so as the molten material is pushed by the following flight passing through the vent. In order to solve such problems, various shapes and forms have been proposed to be applied to the screw and the vent.
Control of the material supply rate has been another try to solve the vent clogging suffered by a conventional vent-type extrusion molding machine or vent-type injection molding machine. The Japanese Patent Publication No. 16820/1980 may be cited as one of such examples wherein the molding machine is provided with a material feeding means adapted to supply material at a constant rate so as to prevent the vent clogging. In a vent-type injection molding machine, the screw length whereby the material is melted changes as the screw retreats in its plastication stroke. This brings about the change in the melt condition of the material, the melting capability of the machine as well as degassing or foaming condition.
The result is the clogging of the vent or instable degassing process or other like gas evacuation problems which could not be solved by, for example, a plastication molding machine as disclosed in said publication which is provided with a material supply means capable of feeding material at a constant rate so as not to feed material in excess.
Conventionally, the material is not supplied to the screw in its injection stroke so that discontinuance in the supplied material is developed in the compression zone or feed zone of the first stage of the screw. Accordingly, different melt conditions of the material result from a plastication stroke wherein the discontinuance developed in the material in the feed zone of the screw is not filled up at the start of the stroke on one hand and from another plastication stroke wherein the material is filled up throughout the feed zone of the first stage on the other.
An attempt may be made to solve such problem of discontinuance developed in the supplied material due to the reciprocal sliding motions of the screw by controlling material supply so as to feed material also in the injection stroke for a certain period of time by means of a timer only to fail because the injection stroke changes as shown, and, therefore, the material still could be supplied in an amount more or less than appropriate. Another problem making proper material supply difficult, as mentioned above, is that the length in the first stage of the screw whereby the material is melted changes as the screw makes reciprocal sliding motions in an axial direction, necessarily resulting in the change of plastication ability of the machine at every moment of the plastication measurement stroke as shown in FIG. 2.
While the plastication capability of a specific screw depends on its shape, its turning speed, the temperature of the heating cylinder and the like, it is preferable to feed material consecutively in the plastication stroke as well as in the injection stroke according to the plastication capability which varies every moment. When material is fed beyond the plastication ability of the machine, the material fills up the space between the screw flights so tightly that only the outer portion of the material is melted while its inner portion is not, as it is forwardly conveyed. As a result, the temperature of the material as a whole is not raised high enough, and degassing and agitating of the material is not fully effected. Further, when material is fed excessively in the plastication stroke, material whose inner portion is not molten is pushed up to the front end of the first stage of the screw so that the material surges or rushes from the first stage into the second stage. More specifically, the first stage is vacant immediately after the material is conveyed to the second stage so material is not conveyed from the first stage to the second stage until the material in the first stage is subsequently melted and carried up to the front end of the first stage in a given amount. Once a given amount of molten material has been conveyed to the front end of the first stage, it surges or rushes into the second stage. When the surge of the material has taken place, the material in the first stage is heated in excess partially, at the front end of the first stage, for example, in case the molten material is not supplied to the second stage. As a result, more gas than usual is generated to develop the vent clogging, the material is discolored or deteriorated. On the other hand, in case material is allowed to pass the first stage quickly, part of the material is not melted enough, resulting in insufficient degassing.
Conversely, when material is supplied in too small an amount in relation to the plastication ability of the screw, the material located at the front ends of the first and second stages is allowed to remain there for a long period of time as it is not followed by a necessary amount of material so that the material is heated in excess, causing vent clogging, discoloration and deterioration of the material. Further, the material newly supplied to the first stage is conveyed forward so fast that it is not melted suffiently. Those phenomena take place in a remarkable manner with the conventional vent-type plastication molding machines. These problems can be solved by supplying material according to the plastication ability of the screw in the plastication and injection strokes so as to fill the cylinder with a fixed amount of material throughout the cylinder. Thus the temperature of the material is raised to a desirable degree, degassing is effected satisfactorily to prevent vent clogging, and good agitation of material is made possible.